Lithium polymer secondary battery

ABSTRACT

A lithium polymer secondary battery which comprises a negative electrode, a positive electrode, and polymer electrolyte layers united respectively with the two electrodes and differing in viscoelastic behavior. In this battery, conformation to the expansion and shrinkage accompanying charge/discharge is easy and the interfacial resistance between each electrode and the polymer electrolyte is kept low.

This application is a continuation of international applicationPCT/JP01/08523 filed 28 Sep. 2001 which designates the U.S.

FIELD OF THE INVENTION

This invention relates to a lithium secondary battery using anion-conductive polymer. More specifically, it relates to a lithiumpolymer secondary battery comprising an anode having an electroactivesubstance comprised of a carbonaceous material capable ofelectrochemically insertion and release of lithium, a cathode having anelectroactive substance comprised of a chalcogenide compound containinglithium, and a polymer electrolyte layer comprised of a matrix of anion-conductive polymer retaining a nonaqueous electrolyte solutiontherein.

BACKGROUND ART

Lithium secondary batteries have a higher energy density in theorycompared to other batteries and thus allow to manufacture a small andlight-weight battery. Therefore, vigorous studies have been focussedthereon to develop a power source of portable electronic instruments.Particularly, performance of such instruments is even increasing inrecent years and their power source is required concominantly therewithto exhibit better discharging characteristics even at a high load. Inorder to fulfill these requirements, various studies are in progressnext to the prior art battery using nonaqueous electrolyte solutionsreferred to as lithium ion battery to develop a battery using a polymerelectrolyte that functions both as the nonaqueous electrolyte solutionand the polymer separator of the prior art battery. Much interest hasbeen focussed to a lithium secondary battery using the polymerelectrolyte because of its remarkable advantages such as the possibilityof making the battery smaller and thinner in size and lighter in weightas well as leak free.

Generally, secondary batteries now available in the market such aslithium secondary batteries make use of a nonaqueous electrolytesolution prepared by dissolving an electrolyte salt in an organicsolvent. The use of this solution is problematic because the solution iseasily susceptible to leakage from the battery parts, dissolution ofelectrode substances or vaporization which may develop problems of longterm reliability, spilling off in the sealing process and the like.

In order to improve these problems, lithium secondary batteries havebeen developed which make use of a polymer electrolyte macroscopicallyoccurring as a solid. The polymer electrolyte consists of a porousmatrix of an ion-conductive polymer impregnated with or retaining anonaqueous electrolyte solution (a lithium salt solution in an aproticpolar organic solvent).

Microscopically the polymer electrolyte has a continuous phase ofnonaqueous electrolyte solution therein and exhibits a high ionconductivity. This results in a low mechanical strength. The mechanicalstrength may be reinforced by including a separator (porous substrate)in the polymer electrode but another problem still remains to exsist.

The lithium secondary battery relies on intercalation or doping oflithium into an electroactive substance which results in expansion andshrinkage of the electroactive layer. If the polymer electrolyte failsto accommodate well the expansion/shrinkage, then physical contactbetween the electrode and the polymer electrolyte will becomeunsatisfactory to develop increased interfacial resistance therebetween.This adversely affect the battery perfomance including discharge andcharge cycle characteristics of the battery.

JP-A-5012913 discloses that the ion conductivity and the elasticity ofthe polymer electrolyte of this type may well be balanced by increasingthe ratio of nonaqueous electrolyte solution to ion-conductive polymerto 200% or higher and also increasing the elasticity and elongation ofthe polymer electrolyte greater than certain levels. Since greaterelasticity levels mean greater strain per unit amount of stress,increased elasticity cannot accommodate both of expansion and shrinkage.In order to accommodate both expansion and shrinkage, it is necessaryfor the polymer electrolyte to have a cushon-like property.

Accordingly, the problem to be solved by the present invention is toprovide a lithium secondary battery including a polymer electrolytelayer that can tolerate expansion and shrinkage of the electroactivesubstance layers and exhibit a buffering effect as a whole.

DISCLOSURE OF THE INVENTION

The present invention provides a lithium polymer secondary batterycomprising an anode having an electroactive substance comprised of acarbonaceous material capable of electrochemically inclusion and releaseof lithium, a cathode having an electroactive substance comprised of achalcogenide compound containing lithium, and a polymer electrolytelayer sandwiched between the cathode and the anode, wherein said polymerelectrolyte layer is divided into a sub-layer integrally formed withsaid cathode and a sub-layer integrally formed with said anode, andwherein the sub-layers exhibit different viscoelastic behavior from eachother.

In a preferred embodiment, the polymer electrolyte sub-layer on thecathode has an elasticity greater than that of the polymer electrolytesub-layer on the anode. This is because the expansion/shrinkage of theelectroactive substance is more remarkable in the cathode than in theanode and, therefore, a greater mechanical strength is required for thepolymer electrolyte sub-layer on the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the discharge capacity at different currentlevels of the battery according to Example 1 of the present invention incomparison with the battery according to Comparative Example 1.

FIG. 2 is a graph showing the discharge curve at a constant current of10 mA of the battery according to Example 1 of the present invention incomparison with the battery according to Comparative Example 1.

FIG. 3 is a graph showing the discharge curve of the battery accordingto Example 1 of the present invention in comparison with the batteryaccording to Comparative Example 1.

BEST MODE FOR CARRYING OUT OF THE INVENTION

The battery of the present invention may be manufactured by forming anion-conductive polymer layer separately on a pre-fabricated cathode andanode and joining the layers together although the manufacturing processis not limited thereto.

Basically, the anode and cathode comprise a current collector in theform of a metal foil and an electroactive substance of the respectiveelectrodes bound with a binder material. The materials of the collectorfoil include aluminum, stainless steel, titanium, copper, nickel and thelike. Aluminum and copper are employed for the cathode and the anode,respectively in consideration of their electrochemical stability,ductility and economy.

Although metal foils are mainly shown herein as the form of anode andcathode collectors, other forms such as mesh, expanded metals, laths,perforated sheets or plastic films having a coating of anelectron-conductive material may be employed although the form ofcollector is not limited thereto.

The electroactive substance of the anode is a carbonaceous matericalcapable electrochemically inclusion and release of lithium. Typicalexamples thereof include particles (flakes, aggregates, fibers,whiskers, beads or ground particles) of natural or artificial graphite.Artificial graphite produced by graphitizing mesocarbon beads, mesophasepitch powder or isotropic pitch powder may also be used.

With regard to the electroactive substance used in the presentinvention, it is more preferable to use as the carbonaceous materialgraphite particles having attached to the surfaces thereof amorphouscarbon particles. These particles may be obtained by dipping thegraphite particles in a coal-based heavy oil such as pitch or apetroleum-based heavy oil and heating recovered graphite particles to atemperature above the carbonizing temperature to decompose the heavyoil, if necessary, followed by milling. Such treatment significantlyretards the decomposing reaction of the nonaqueous electrolyte solutionand the lithium salt occurring at the anode during the charge cycle toenable the charge and discharge cycle life to be improved and also thegas evolution due to the above decomposition reaction to be prevented.In the above carbonaceous material, micropores contributing to increasein BET specific surface area have been filled with the attached carbonparticles derived from the heavy oil. The specific surface area thereofis generally below 5 m²/g, preferably in the range between 1 to 5 m²/g.Greater specific surface areas are not preferable because increasedcontacting surface area with the ion-conductive polymer makes undesiredside reactions to be taken place more easily.

The cathodic electroactive substance to be used in the present inventionin conjunction with the carbonaceous anodic active substance ispreferably selected from a composite oxide of laminar or spinelstructure represented by the formula: Li_(a)(A)_(b)(B)_(c)O₂ wherein

A is a transition metal element;

B is an element selected from the group consisting of a non-metal orsemi-metal element of group 3B, 4B and 5B of the periodic chart, analkaline earth metal, Zn, Cu and Ti;

a, b and c are numbers satisfying the following relationship:0<a≦1.150.85≦b+c≦1.30, andc>0

Typical examples of the composite oxides include LiCoO₂, LiNiO₂ andLiCoxNi_(1·x)O₂ (0<x<1). Use of these compounds in conjunction with acarbonaceous material as an anodic electroactive substance isadvantageous in that the battery exhibits a practically acceptabledynamic voltage even when the voltage variation generated by chargingand discharging the carbonaceous material per se (about 1 volt vs.Li/Li⁺), and that lithium ions necessary for charging and dischargingthe battery are already contained in the form of, for example, LiCoO₂ orLiNiO₂ before assembling the battery.

When preparing the anode and cathode, the respective electroactivesubstances may be combined, where necessary, with a chemically stableconductor material such as graphite, carbon black, acetylene black,carbon fiber or conductive metal oxides to improve the electronconductivity thereof.

The binder is selected among those thermoplastic resins which arechemically stable, soluble in a suitable solvent but hardly attackedwith the nonaqueous electrolyte solution. A variety of suchthermoplastic resins have been known. For example, polyvinylidenefluoride (PVDF) may preferably used since this resin is selectivelysoluble in N-methyl-2-pyrrolidone. Other examples of usablethermoplastic resins include polymers and copolymers of acrylonitrile,methacrylonitrile, vinyl fluoride, chloroprene, vinyl pyridine and itsderivatives, vinylidene chloride, ethylene, propylene and cyclic dienes(e.g. cyclopentadiene, 1,3-cyclohexadiene). A dispersion of the binderresin may also be used in place of a solution.

The electrode may be produced by kneading the respective electroactivesubstances and, where necessary, the conductor material with a solutionof the binder resin to prepare a paste, applying the paste on a metalfoil using a suitable coater to form a film of uniform thickness, andcompressing the film after drying. The proportion of the binder resin inthe electroactive substance layer should be minimum and generally liesfrom 1 to 15% by weight. The proportion of the conductor materialusually lies, when used, from 2 to 15% by weight of the electroactivesubstance layer.

The polymer electrolyte layer is formed on the respective electroactivesubstance layers thus prepared integrally therewith. The polymerelectrolyte layer is comprised of a matrix of an ion-conductive polymerimpregnated with or retaining a nonaqueous electrolyte solutioncontaining a lithium salt. The polymer electrolyte layer occursmacroscopically in a solid state but microscopically retains acontinuous phase of the lithium solution formed therein in situ. Thepolymer electrolyte layer of this type has an ion-conductivity higherthan that of the corresponding polymer electrolyte free from the lithiumsolution.

The polymer electrolyte layer may be formed by polymerizing (heatpolymerization, photopolymerization etc.,) a precursor monomer of theion-conductive polymer in the form of a mixture with the nonaqueouselectrolyte solution containing a lithium salt.

The monomer component of the above mixture which can be used for thispurpose should include a polyether segment and also be polyfunctional inrespect to the polymerization site so that the resulting polymer forms athree dimensional crosslinked gel structure. Typically, such monomersmay be prepared by esterifying the terminal hydroxyl groups with acrylicor methacrylic acid (collectived called “(meth)acrylic acid”). As iswell known in the art, polyether polyols are produced byaddition-polymerizing ethylene oxide (EO) alone or in combination withpropylene oxide (PO) using an initiator polyhydric alcohol such asethylene glycol, glycerine or trimethylolpropane. A monofunctionalpolyether polyol (meth)acrylate may be used in combination withpolyfunctional monomers.

The poly- and monofunctional monomers are typically represented by thefollowing general formulas:

wherein R₁ is hydrogen or methyl;

A₁, A₂ and A₃ are each a polyoxyalkylene chain containing at least 3ethylene oxide (EO) units and optionally some propylene oxide (PO) unitssuch that PO/EO=0–5 and EO+PO≧35.

wherein R₂ and R₃ are hydrogen or methyl;

A₄ is a polyoxyalkylene chain containing at least 3 EO units andoptionally some PO units such that PO/EO=0–5 and EO+PO≧10.

wherein R₄ is a lower alkyl, R₅ is hydrogen or methyl, and A₅ is apolyoxyalkylene chain containing at least 3 EO units and optionally somePO units such that PO/EO=0–5 and EO+PO≧3.

The nonaqueous electrolyte solution is prepared by dissolving a lithiumsalt in a nonpolar, aprotic organic solvent. Non-limitative examples ofthe lithium salt solutes include LiClO₄LiBF₄, LiAsF₆, LiPF₆, LiI, LiBr,LiCF₃SO₃, LiCF₃CO₂, LiNC(SO₂CF₈)₂, LiN(COCF₃)₂, LiC(SO₂CF₃)₂, LiSCN andmixtures thereof.

Non-limitative examples of the organic solvents include cyclic carbonateesters such as ethylene carbonate (EC) or propylene carbonate (PC);straight chain carbonate esters such as dimethyl carbonate (DMC),diethyl carbonate (DEC) or ethyl methyl carbonate (EMC); lactones suchas γ-butyrolactone (GBL); esters such as methyl propionate or ethylpropionate; ethers such as tetrahydrofuran and its derivatives,1,3-dioxane, 1,2-dimethoxyethane, or methyl diglyme; nitrites such asacetonitrile or benzonitrile; dioxolane and derivatives thereof;sulfolane and derivatives thereof; and mixtures of these solvents.

Since the polymer electrolyte on the electrode, particularly on thecarbonaceous material of the anode is required to contain a nonaqueouselectrolyte solution of which side reactions with the graphite-basedcarbonaceous material are retarded, it is preferable to use a solventsystem consisting primarily of EC and another solvent selected from PC,GBL, EMC, DEC or DMC. For example, a nonaqueous electrolyte solutioncontaining 3 to 35% by weight of a lithium salt dissolved in the abovesolvent mixture containing 2 to 50% by weight of EC exhibits asatisfactory ion conductivity even at low temperatures.

The proportion of the nonaqueous solution in the mixture with theprecursor monomer should be large enough to maintain the solution ascontinuous phase in the crosslinked polymer electrolyte layer but shouldnot be so excessive to undergo phase separation and bleeding of thesolution from the gel. This can be accomplished by the ratio of themonomer to the electrolyte solution generally within a range from 30/70to 2/98, preferably within a range from 20/80 to 2/98 by weight.

The polymer electrolyte layer may optionally include a porous substrateas a support member. Such substrate may be either a microporous membranemade from a polymer which is chemically stable in the nonaqueouselectrolyte solution e.g. polypropylene, polyethylene or polyester, or asheet (i.e. paper or nonwoven fabric) made from fiber of such poymers.It is preferable, that the substrate has a air permeability from 1 to500 sec./cm³ and can retain the polymer electrolyte therein at asubstrate: polymer electrolyte ratio from 91/9 to 50:50. This isnecessary to achieve an optimum balance between the mechanical strengthand the ion conductivity.

When the substrate is not used, the polymer electrolyte layer integralwith the respective electrodes may be fabricated by casting the mixtureof the precursor monomer and the nonaqueous electrolyte solution on therespective electroactive substance layers to form a film andpolymerization the monomer in situ. Then both electrodes are joinedtogether with their polymer electrolyte layers facing inwardly.

When used, the substrate is applied on the electroactive substance layerof either one of the electrodes. Then the mixture of the precursormonomer and the electrolyte solution is cast on the substrate followedby polymerization of the monomer in situ to form the polymer electrolytelayer integral with the substrate and the electrode. This electrode isjoined together with the other electrode including the polymerelectrolyte layer free of the substrate formed as above with theirpolymer electrolyte layers facing inwardly.

The above methods are preferred since they insure to form the polymerelectrolyte layer integral with the electrode and the substrate, whenused, in a simple manner.

The mixture of the precursor of ion-conductive polymer (monomer) and thenonaqueous electrolyte solution containing a lithium salt contains asuitable polymerization initiator depending on the polymerizationmethod, e.g. a peroxide type or azo type initiator for heatpolymerization and a photoinitiator such as acetophenone, benzophenoneor phosphine series for photopolymerization. The polymerizationinitiator may be used in an amount from 100 to 1,000 ppm and should notbe used in excess.

According to the present invention, the polymer electrolyte layers(sub-layers), formed on the cathode and anode, respectively havedifferent viscoelastic behavior from each other. Generally the polymerelectrolyte is a viscoelastic mass having both properties similar to agenuine elastic solid in which the amount of strain is proportional tothe amount of stress applied and properties similar to a Newton'sviscous liquid in which the speed of deformation is proportional to theamount of stress applied. Therefore, when the polymer electrolyte istested for the stress-strain (elongation) relationship at a constanttensile speed, it behaves like a elastic solid until it reaches ayielding point and thereafter it behaves like a viscous liquid so thatit deforms largely in response to a small increase in the amount ofstress before break. In this test, the larger the amount of exertedenergy before break, the larger the toughness of the test material. Theabove amount of energy may be estimated from the modulus of elasticityof the test material.

In case of the polymer electrolyte, the modulus of elasticity is afunction of the crosslinking density thereof. Therefore, gelled polymerelectrolytes having different modulus of elasticity levels may beproduced, for example, by varying monomer compositions of theion-conductive polymer at a constant ratio of matrix polymer/nonaqueouselectrolyte solution. Accordingly, a polymer gel having a relativelyhigh modulus of elasticity may be obtained, for example, by selecting amonomer composition including a relatively large proportion of a monomerhaving a large number of functionality with regard to polymerizablegroups. This method is merely one of preferred exemplifying methods andother methods for producing polymer electrolytes having differentviscoelastic properties would be obvious to one skilled in the art.

In a preferred embodiment, the polymer electrolyte sub-layer on thecathode has a modulus of elasticity in a range from 10⁴ to 10⁶ dyne/cm²and the polymer electrolyte sub-layer on the anode has a modulus ofelasticity in a range from 10³ to 10⁶ dyne/cm² and the modulus ofelasticity of the sub-layer on the cathode is at least 10% greater thanthat of the sub-layer on the anode. The polymer electrolyte layer as awhole can accommodate well the expansion and shrinkage of electroactivesubstances under the above conditions.

EXAMPLE

The following Examples are for illustrative purpose only and notintended to limit the scope of the present invention thereto.

Example 1

1) Fabrication of Anode

As a carbonaceous material, a particulate graphite having amorphouscarbon microparticles attached to the surfaces of graphite particles wasused. The graphite particles have a d002 value of 0.336 nm determinedaccording to the large angle X-ray diffraction method, an Lc value of100 nm, an La value of 97 nm and a BET specific surface area of 2 m²/g.

A blend of 100 weight parts of the above carbonaceous material and 9weight parts of polyvinylidene fluoride (PVDF) was kneaded with anamount of N-methyl-pyrrolidone (NMP). The resulting paste was appliedonto a rolled copper foil of 20 μm thickness, dried and compressed. Thesurface area of the cathode was 9 cm² and the thickness thereof was 85μm.

2) Nonaqueous Electrolyte Solution/Monomer Mixture

LiBF₄ was dissolved at a concentration of 1.0 mol/L in a mixture ofethylene carbonate (EC):propylene carbonate (PC): γ-butyrolactone (GBL):ethyl methyl carbonate (EMC)=30:20:30:20 by volume.

To 95 weight parts of this solution were added 2.5 weight parts of atrifunctional polyether polyol triacrylate (MW=7500–9000) of theformula:

wherein A₁, A₂ and A3 are each polyoxyalkylene chain containing at least3 EO units and at least one PO unit in PO/EO ratio of 0.25; and 2.5weight parts of triethylene glycol methyl ether acrylate of the formula:

Then 1,000 ppm of 2,2-dimethoxy-2-phenylacetophenone (DMPA) was added toprepare a polymerization liquid.

3) Fabrication of Polymer Electrolyte Layer Integral with Anode andSeparator Substrate

The above polymerization liquid was cast on the electroactive substancelayer of the anode.

A polyester nonwoven fabric having an air permeability of 380 sec/cm³, athickness of 20 μm and an area of 10 cm² was placed on the anode and theabove polymerization liquid was poured thereon in an amount sufficientto reach a fabric: liquid ratio=90:10 by weight. Then the anode-fabricstack was irradiated with UV radiation of 365 nm wavelength at anintensity of 30 mW/cm² for 3 minutes to form a gelled polymerelectrolyte layer integrally with the anode and the nonwoven fabric. Thethickness of the polymer electrolyte layer was 20 μm.

4) Fabrication of Anode

100 weight parts of LiCoO₂ having an average particle size of 7 μm and 5weight parts of acetylene black, and 5 weight parts of PVDF were blendedand kneaded with an amount of NMP. The resulting paste was applied on arolled aluminum foil of 20 μm thickness, dried and compressed. The areaand the thickness of cathode were 80 μm and 9 cm², respectively.

5) Polymerization Liquid on Cathode

LiBF₄ was dissolved at a concentration of 1.0 mol/L in a mixture ofethylene carbonate (EC): propylene carbonate (PC): γ-butyrolactone(GBL): ethyl methyl carbonate (EMC)=30:20:30:20 by volume.

To 90 weight parts of this solution were added 5 weight parts of atrifunctional polyether polyol triacrylate as used on the anode side and1,000 ppm of DMPA to obtain a polymerization liquid.

6) Fabrication of Polymer Electrolyte Layer on Cathode

The above polymerization liquid was cast on the cathode and irradiatedwith UV radiation of 365 nm wavelength at an intensity of 30 mW/cm² forthree minutes to fabricate a polymer electrolyte layer integrally withthe cathode. The thickness of the polymer electrolyte layer was 10 μm.

6) Assembly of Battery.

The cathode and the anode as prepared above were joined together withtheir polymer electrolyte layers facing inwardly to assemble a batteryhaving a total thickness of 190 μm.

Comparative Example 1

Example 1 was repeated except that the polymer electrolyte sub-layer ofExample 1 was used in both of the cathode and the anode.

The polymer electrolyte sub-layer of the anode in Example 1 (same as thepolymer electrolyte sub-layers on the cathode and the anode inComparative Example 1) was tested for the modulus of elasticityaccording to the method described below. The same test was alsoconducted for the cathode polymer electrolyte sub-layer on the cathodeof Example 1.

The respective precursor liquids were cast on a stainless steel foil andirradiated with UV radiation of 365 nm wavelength at an intensity of 30mW/cm² for 3 minutes to form a gelled polymer electrolyte sheet having athickness of 500 μm. A sample was taken from this sheet and the modulusof elasticity thereof was determined using a dynamic viscoelastometer.The results are shown in Table 1 below.

Evaluation of Battery Performance:

The batteries of Example 1 and Comparative Example 1 were charged at aconstant current of 4.0 mA until the battery voltage reached 4.1 V andthe charged at a constant voltage for a total pre-charge time of 12hours. The charged batteries were then discharged at different constantcurrent levels of 2, 3, 5, 10 and 20 mA until the battery voltagedecreased to 2.75 V. The discharge capacities at different dischargecurrent levels are shown in the graph of FIG. 1.

Similarly, the batteries were charged under same conditions as above andwere discharged at a constant current level of 10 mA until the batteryvoltage decreased to 2.75 V. The discharge curves of the respectivebatteries are shown in the graph of FIG. 2.

Similarly, the batteries were charged under same condition as above andthen discharged at a constant current level of 2.3 mA until the batteryvoltage reached 2.75 V. This charge-discharge cycle was repeated anumber of times for testing the variation in discharge capacity. Theresults are shown in the graph of FIG. 3.

As the results of these tests show, the battery of Example 1 is superiorto the battery of Comparative Example 1 in terms of the dischargecapacity at different current levels, the discharge capacitycharacteristics under a high load and the charge-discharge cyclingcharacteristics. It is postulated that these results are attributed tothe fact that the polymer electrolyte layer of the battery of Example 1as a whole can easily accommodate the expansion/shrinkage of the cathodeand anode during the charge and discharge cycles compared to the polymerelectrolyte layer of the battery of Comparative Example 1 to keep theinterfacial resistance between the polymer electrolyte layer and theelectrode at a minimum level.

Separately, the batteries of Example 1 and Comparative Example 1 weretested, respectively for the number of incident of microshort circuit in20 batteries immediately after manufacutre. The results are also shownin Table 1. It is also postulated that the fewer number of incident ofmicroshort circuit in the battery of Example 1 compared to the batteryof Comparative Example 1 is attributed to the improved mechanicalproperty of the polymer electrolyte layers a whole.

TABLE 1 Example 1 Comp. Ex. 1 Item Cathode Anode Cathode Anode Modulusof 2.18 × 10⁵ 8.9 × 10³ 8.9 × 10³ 8.9 × 10³ elasticity, 25° C.(dyne/cm²) Incident of 0/20 2/20 microshort circuit

1. A lithium secondary battery comprising an anode having anelectroactive substance comprised of a carbonaceous substance capable ofelectrochemically inclusion and release of lithium, a cathode having anelectroactive substance comprised of a chalcogenide compound containinglithium, and a polymer gel electrolyte layer sandwiched between thecathode and the anode, wherein said polymer gel electrolyte layercomprises a matrix of an ion-conductive polymer of polyether polyol(meth)acrylate retaining a nonaqueous electrolyte solution therein,wherein said polymer gel electrolyte layer is divided into a firstsub-layer integrally formed with said cathode and a second sub-layerintegrally formed with said anode, and wherein the precursor monomer ofsaid ion-conductive polymer in said first sub-layer has a polymerizablefunctionality of 3, and the precursor monomer of said ion-conductivepolymer in said second sub-layer has a polymerizable functionality lessthan 3, the precursor monomer of said ion-conductive polymer in saidfirst sub-layer being a trihydric polyether polyol triacrylate and theprecursor monomer of said ion-conductive polymer in said secondsub-layer being a mixture of said trihydric polyether polymertriacrylate with triethylene glycol monomethyl ether acrylate at aweight ratio of 1:1.
 2. The lithium polymer secondary battery accordingto claim 1 wherein the solvent of said nonaqueous electrolyte solutionis selected from ethylene carbonate, propylene carbonate, ethyl methylcarbonate, dimethyl carbonate, diethyl carbonate, r-butyrolactone or amixture thereof.
 3. The lithium polymer secondary battery according toclaim 1 wherein said electroactive substance of said anode is aparticulate graphite having amorphous carbon attached to the surfacesthereof.
 4. The lithium polymer secondary battery according to claim 1wherein said polyether polyol poly(methacrylate) contains a plurality ofpolyether chains consisting of an ethylene oxide (EO) unit andoptionally a propylene oxide (PO) unit.